Animal Kingdom: Characteristics, Classification & Systems
The Animal Kingdom
Characteristics of Animals
Animals are organisms that share several key characteristics:
- Eukaryotes: Their cells have a nucleus and other membrane-bound organelles.
- Heterotrophs: They obtain nutrients by consuming other organisms.
- Multicellular: They are made up of multiple cells.
- Respond quickly to stimuli: They have nervous systems that allow them to react to their environment.
- Actively moving: Most animals are capable of movement.
- Have sexual reproduction: They reproduce sexually, with the fusion of gametes.
- Limited growth: They typically reach a certain size and stop growing.
Animals are broadly divided into two main categories: vertebrates and invertebrates. They can also be classified based on their diet as herbivores (plant-eaters), carnivores (meat-eaters), or omnivores (both plant and meat-eaters).
Invertebrate Animals
Lower Invertebrates
- Porifera (Sponges): Aquatic, sac-shaped, fixed to the substrate, have an internal skeleton (e.g., Spongia).
- Coelenterates (Cnidarians): Radial symmetry, live fixed to the substrate as polyps (e.g., jellyfish, corals).
- Platyhelminthes (Flatworms): Flat and elongated body, bilateral symmetry (e.g., tapeworm).
- Nematodes (Roundworms): Elongated cylindrical bodies, bilateral symmetry (e.g., nematodes).
Higher Invertebrates
- Annelids (Segmented Worms): Segmented body, organs and systems, bilateral symmetry (e.g., earthworms).
- Mollusks: Soft body, with a locomotor organ called a foot, bilateral symmetry (e.g., slugs, snails, clams).
- Arthropods: Segmented body (head, thorax, abdomen), jointed appendages, hard exoskeleton, bilateral symmetry (e.g., insects, crabs, spiders).
- Echinoderms: Spines, radial symmetry (e.g., sea urchins, starfish).
Vertebrate Animals
- Fish: Aquatic, scales (bony or cartilaginous), fins, bony skeleton, breathe through gills, external fertilization.
- Amphibians: Aquatic larvae (tadpoles), terrestrial adults, two pairs of limbs, bare skin, breathe through gills (larvae) and lungs (adults).
- Reptiles: Scales, skin covered by shields and plaques, two pairs of limbs (except snakes), breathe through lungs.
- Birds: Feathers, wings, two legs, breathe through lungs.
- Mammals: Terrestrial or aquatic, hair, two pairs of legs, breathe through lungs.
Digestion
Digestion is the process by which animals obtain nutrients from food. There are two main levels of digestion:
- Cellular Level: Some animals have tissues with specific cells that capture and digest food.
- Organ Level: Most animals have specialized organs or organ systems responsible for digestion.
Respiration
Respiratory structures in animals have the following characteristics:
- Formed by very thin membranes.
- Highly vascular (lots of blood flowing through them) and very humid.
- Divided to increase the surface area for gas exchange.
Respiration is the process of oxidizing food to release energy. This involves breaking down complex molecules into simpler ones, allowing the release of energy for the animal’s vital functions. Respiration is also called biological oxidation.
Circulation
Animals have circulatory systems to transport nutrients, oxygen, and waste products throughout their bodies. There are two main types of circulatory systems:
- Pulmonary Circulation: Starts in the right ventricle of the heart and ends in the left atrium. Blood carrying carbon dioxide travels through the pulmonary artery to the lungs, where gas exchange occurs. Oxygenated blood then returns to the heart through the pulmonary veins.
- Systemic Circulation: Starts in the left ventricle and ends in the right atrium. Oxygenated blood travels through the aorta to the body’s cells, where gas exchange occurs. Deoxygenated blood then returns to the heart through the vena cava.
The process of blood oxygenation in the lungs is called hematosis.
Hearts
Different animal groups have different types of hearts:
- Fish: 2 chambers, simple, closed, complete.
- Amphibians: 3 chambers, double, closed, incomplete.
- Reptiles: 4 chambers, double, closed, partially incomplete.
- Birds and Mammals: 4 chambers, double, closed, complete.